UNIT 4: THE ATMOSPHERE.
1. COMPOSITION AND STRUCTURE OF THE ATMOSPHERE. WEATHER AND CLIMATE
1.1 The composition of the atmosphere.
The atmosphere is a layer of gases that surrounds the Earth. The most important gases
in the atmosphere are: nitrogen (78%); oxygen (21%); argon (0.9%); carbon dioxide
(0.03%); water vapour and other gases such as ozone.
We call this mixture of gases air. The air we breathe is composed of: the original natural
components and additional gases and smoke from industry, cars… Air is also present in
the ground, and some gases are dissolved in water.
1.2 How did the atmosphere form?
The Earth’s atmosphere formed billions of years ago in several stages:
First, volcanoes emitted the following gases: water vapour, carbon dioxide, ammonia
(which contains nitrogen atoms), methane.
Then, the water vapour cooled and condensed to form oceans.
Finally, life appeared in the oceans: the first living things were bacteria. Later, some of
the bacteria started to perform photosynthesis, which converted carbon dioxide into
oxygen.
1.3 The structure of the atmosphere.
At different altitudes, the atmosphere has different characteristics such as temperature
and pressure. Scientists therefore divide the atmosphere into five layers.
The exosphere (500-800 Km) is the outer part of the atmosphere. It is where most
artificial satellites orbit the Earth.
The thermosphere: From 80 km up to 500 km in height. It is a hot layer with very low
gas density. This is also called Ionosphere. It is where the aurora borealis is produced.
Its temperature increases to 1000ºC due to X-rays and gamma rays (harmful solar
radiation) from the Sun. Comets appear here.
The mesosphere: From 50 km up to 80 km in height. It contains clouds of ice and dust.
The temperature decreases as altitude increases (it gets colder the higher you go). The
upper part of the mesosphere is very cold.
The stratosphere: From 12 km up to 50 km. It contains the ozone layer. The ozone
absorbs ultraviolet radiation. This means that from about 20 km to 50 km, the
temperature increases as altitude increases (-60 ºC- 0ºC). Jet planes fly in the lowest
part of the stratosphere. At this altitude, the density of air is too low for humans to
survive, so the inside of the plane has to be pressurized (kept at the right pressure).
The troposphere: About 12 km thick. It is the layer closest to the Earth, where we live.
The air is dense (It contains 80% of the air in the atmosphere). It is also where weather
occurs (clouds, rain, snow, wind, etc). The temperature decreases as altitude increases
(from 20ºC to -60ºC)
1.4 The weather and the climate.
Atmospheric weather is the state of the atmosphere at a certain time and at a certain
place. It is the short-term change in things like temperature, precipitation and wind.
It refers to a series of atmospheric phenomena. We describe atmospheric weather in
terms of: sun, rain, maximum and minimum temperatures, clouds, winds, and fronts.
Meteorologists take all the necessary measurements to predict the weather at a certain
place.
The climate is a set of meteorological phenomena which characterize a certain region
for a long period of time. It is the long-term average weather conditions.
It is determined by:
 Latitude. How far north of south a place is from the Equator. At the Equator,
solar energy is concentrated and causes high temperatures. Toward the north
and south, solar energy is more spread out and causes cold temperatures.
 Distance from the sea. Sea water heats up slowly and cools down slowly. In
winter the sea releases heat, keeping coastal areas warmer. Land heats up
quickly and cools down quickly. In summer, cold sea keeps coastal areas cooler.
 Altitude. The height above sea level. The higher a place is, the colder it will be.
 Ocean currents. Warm ocean currents flow up from the tropics to the poles and
warm up surrounding areas, especially in winter. Cold ocean currents can lower
temperatures in an area.
Climate is defined in terms of temperature and precipitations throughout the year
and it is also defined by establishing the average of both temperature and
precipitations over several years (at least 30 years).
2. THE IMPORTANCE OF THE ATMOSPHERE.
2.1 Temperature regulation.
During the day, the Earth’s surface gets hot due to solar radiation. The heated ground
releases that heat back in the air, emitting infrared radiation which heats the
troposphere upwards.
Some atmospheric gases, such as CO2, keep most of that heat, just like the glass walls in
a greenhouse do. Thanks to this phenomenon, called the greenhouse effect, the
average temperature of the Earth is about 15 ºC and it does not cool down during the
night. On other solar bodies, like the Moon, which do not have an atmosphere, the
temperature falls below -30ºC during the night.
2.2 The atmosphere as a shield.
 The atmosphere protects us from harmful radiation.
The Sun emits radiation, such as gamma rays, X-rays and ultraviolet rays harmful
to life. Some gases in the atmosphere absorb this harmful radiation and
transform it into heat. This is called the filter effect.

It protects us from the impact of meteorites.
If there was no atmosphere, the Earth’s surface would be hit by rocks coming
from outer space, attracted by the Earth’s gravity. When these rocks hit the
atmosphere at a very high speed, the friction against gases makes the rocks so
hot that they become incandescent and disintegrate before touching the ground.
Only the biggest rocks (which are very rare) can get through our gas shield.
2.3 The atmosphere and living things.
The atmosphere is essential for living things. For example, photosynthetic organisms use
CO2 to make their nutrients and almost all living things use oxygen (O2) breathe. Without
the greenhouse effect the planet’s temperature would not be adequate for life. Human
beings also use the wind’s energy to move sailing boats, generate electricity and power
wind mills.
Fuentes: Natural Sciences (Anaya , Oxford, Santillana y Lingua Frame)